This invention relates in general to pickling and more particularly to a method of dry pickling. Pickling is the process of chemically removing oxides and scale from the surface of a metal, conventionally by the action of water solutions of inorganic acids. While acid pickling is only one of several methods of removing undesirable surface oxides, it is the most widely used in the manufacture of sheet and tin mill products, because of comparatively low operating costs and ease of operation. Considerable variation in type of pickling solution, operation, and equipment is found in the industry. Among the types of pickling equipment may be mentioned batch picklers, modified batch, semi-continuous and continuous picklers.
With the advent of continuous cold reduction mills, it was necessary to design and develop suitable equipment to remove the oxides resulting from the continuous hot-rolling operation and prepare the hot rolled breakdowns for cold reduction in coil form. This operation is performed in a continuous pickling line. The primary function of a continuous pickling line, as of other pickling processes, is the removal of oxide from the steel surface. This serves to promote maximum reduction with a minimum of power, to assure good roll life in the cold reduction mills and to secure the increased surface density possible with cold working. Modern continuous pickling lines operate at speeds as high as 700 to 800 ft. per min.
Prior to our invention, the pickling zone consisted of several individual acid-proof tanks located in a series, comprised of an effective immersion length of about 250 to 300 ft. While most lines have from three to five tanks, each about 70 to 80 ft. long, some modern lines have only one long tank, divided by weirs into four or five sections, thereby increasing effective immersion depth about 10 to 15 percent. The inside dimensions of these tanks have been more or less standardized at 4 ft. in depth and about 1 ft. wider than the maximum product width. A steel sheel is used for support with layers of rubber bonded to the steel and the rubber is protected from abrasion by a lining of about 9 in. of silica-base acid-proof brick. For operating temperatures in excess of 200.degree.F, a bakelite-base cement generally is used for bonding. In modern high-speed lines operating at 200.degree. to 220.degree.F, the brick facing gradually is eroded away, so that replacement is required after several years of operation. Occasionally, small leaks in the rubber lining and the steel tank require patching.
Underpickling results when the steel has not had sufficient time in the pickling tanks to become free of adherent scale and occurs when acid concentration, solution temperatures and line speed are not balanced properly. Variations in the oxide and composition of the steel are also factors in underpickled product, as well as such factors as coiling temperature off the hot strip mill and inadequate amount of cold working through the processor. Overpickling results from line delays which permit sections of the steel to remain in the acid too long. The presence of an inhibitor reduces iron loss, but when an inhibitor is not used, iron loss during a short delay period appreciably reduces thickness of the steel and raises the hazard of hydrogen embrittlement. Pitting is related to overpickling, the presence of non-metallic inclusions near the steel surface, and to rolled-in scale, slag or a refractory substance. While overpickling is not common in continuous pickling operations, its occurrence does have a very serious effect on cold reduction performance and surface appearance of the finished product. Product damage from handling or improper equipment adjustment can render the steel unsuitable for further processing.
Conventional pickling processes employing acids generally create extremely difficult waste disposal problems. Spent pickle liquors are highly destructive and undesirable. At present, one of the more economical and environmentally acceptable disposal methods is to dump the waste pickle liquor into deep wells; in some cases, however, the application of considerable pressure is necessary to see that the fluid is dispersed into the underground strata. But little is known about the ultimate disposition and effects of deep-well waste disposal, and it is generally thought that such methods of disposal should be discontinued if a suitable alternative is found.
U.S. Pat. No. 2,619,434 to Kraus teaches that mixtures of carbon monoxide and chlorine may be used to remove oxide scales from certain metals. In particular, Kraus uses mixtures of chlorine and carbon monoxide to remove scale from reaction zones deposited during a vapor phase reaction, particularly of a titanium halide with an oxidizing gas. In example 3, Kraus compares his chlorine/carbon monoxide mixture with the use of chlorine alone, finding his mixture far superior. This scale, which is formed in a vapor phase oxidation of chlorides and not formed from a substrate, is quite different from the type of scale which is subject to migration of ions such as iron ions through the substrate to the scale, which are capable of changing the valence state of the iron.
Cone et al, in U.S. Pat. No. 2,625,495, also uses a combination of two gases to treat the scale. The first gas used by Cone is always an oxidizing gas and chlorine may be used as the second gas. In ths procedure Cone appears to require a thin oxide layer and one which contains carbonaceous material, rather than a thick scale layer of the type one obtains from hot rolled material. Cone does not obtain enough carbon from the carbonaceous material to reduce a thick scale, i.e., generally more than 1/10,000 inch thick, such as those obtained from hot strip.
Upon carefully examining the patents of Kraus and Cone, we find that both refer to the use of chlorine gas (or Helgas) but impose the additional condition that carbon monoxide or a carbon monoxide precursor (such as carbon or rolling oils on the strip) or other reducing agent such as hydrogen must also be present to effect the removal of the particular oxides treated. This reaction, the chlorination of oxides in a reducing atmosphere, is a very rapid and thermodynamically favorable one going essentially to completion according to the following equation as applied to Fe.sub.2 O.sub.3 at 1250.degree.F (675.degree.C or 950.degree.K):
The pickling processes which require BOTH carbon monoxide and chlorine for carbon steels: EQU Fe.sub.2 O.sub.3 + 3CO + 2Cl.sub.2 .fwdarw. 2FeCl.sub.2 + 3CO.sub.2 ##EQU1##
Where K is the equilibrium constant at 950.degree.K ##EQU2## where a's are the activities (concentrations) of the components used in this reaction. The equilibrium is decidedly in favor of the reaction, as written, and the reaction should go to completion to form FeCl.sub.2 and CO.sub.2.
For the process as practiced on stainless steels the following equation applies: EQU Cr.sub.2 O.sub.3 + 3CO + 2Cl.sub.2 .fwdarw. 2CrCl.sub.2 + 3CO.sub.2 ##EQU3## EQU or log K = 15.3 Again the reaction is in favor of the reaction as written and should go to completion to form CrCl.sub.2 and CO.sub.2. These are the chemical reactions which form the basis of the patents of Cone, Turin and Kraus.
The reader may also be interested in the pickling methods discussed by Kuhn in U.S. Pat. No. 3,544,368, Bartek in U.S. Pat. No. 3,467,549 and Singer in U.S. Pat. No. 3,529,998.